Environmental concerns have led to continued efforts to reduce the CO, hydrocarbon and nitrogen oxide (NOx) emissions of compression ignited (diesel-fueled) and spark ignited (gasoline-fueled) light duty internal combustion engines. Further, there have been continued efforts to reduce the particulate emissions of compression ignited light duty internal combustion engines. To meet the upcoming emission standards for passenger cars, original equipment manufacturers (OEMs) will rely on the use of additional exhaust gas after-treatment devices. Such exhaust gas after-treatment devices may include catalytic converters, which can contain one or more oxidation catalysts, NOx storage catalysts, and/or NH3 reduction catalysts; and/or a particulate trap.
Oxidation catalysts can become poisoned and rendered less effective by exposure to certain elements/compounds present in engine exhaust gasses, particularly by exposure to phosphorus and phosphorus compounds introduced into the exhaust gas by the degradation of phosphorus-containing lubricating oil additives. Reduction catalysts are sensitive to sulfur and sulfur compounds in the engine exhaust gas introduced by the degradation of both the base oil used to blend the lubricant, and sulfur-containing lubricating oil additives. Particulate traps can become blocked by metallic ash, which is a product of degraded metal-containing lubricating oil additives.
One of the most effective antioxidant and antiwear agents, from both a performance and cost-effectiveness standpoint, used conventionally in lubricating oil compositions for internal combustion engines comprises dihydrocarbyl dithiophosphate metal salts. The metal may be an alkali or alkaline earth metal, or aluminum, lead, tin, molybdenum, manganese, nickel or copper. Of these, zinc salts of dihydrocarbyl dithiophosphate (ZDDP) are most commonly used. While such compounds are particularly effective antioxidants and antiwear agents such compounds introduce phosphorus, sulfur and ash into the engine that can shorten the service life of exhaust gas after-treatment devices, as described supra.
To insure a long service life, lubricating oil additives that exert a minimum negative impact on such after-treatment devices must be identified, and OEM specifications for “new service fill” and “first fill” lubricants are expected to require maximum sulfur levels of 0.30 wt. %; maximum phosphorus levels of 0.08 wt. %, and sulfated ash contents below 0.80 wt. %. Such lubricating oil compositions can be referred to as “low SAPS” (low sulfated ash, phosphorus, sulfur) lubricating oil compositions.
At the same time, the selected lubricating oil composition must provide adequate lubricant performance, including adequate wear protection and detergency. The performance of lubricating oil compositions used in light duty spark and compression-ignited vehicular internal combustion engines, specifically antiwear and engine cleaning performance, must be maintained at the high level dictated by the “new service”, and “first fill” specifications of the OEM's.
European patent application 1 167 497 A2 discloses a low SAPS lubricating oil composition comprising an ashless dispersant with a certain nitrogen content, a metal-containing detergent containing an organic acid metal salt selected from the group comprising an alkali or alkaline earth metal salt of an alkyl salicylic acid and an alkali or alkaline earth metal salt of an alkylphenol providing a certain sulphated ash content, a zinc dialkyldithiophosphate providing a specified phosphorus amount and an oxidation inhibitor. Lubricating oils formulated in accordance with this patent application are stated to exhibit good high temperature detergency despite the lower sulfur, phosphorus and sulphated ash levels of the compositions.